It is known to provide a Passive Optical Network that allows a single optical fibre to serve multiple end users. Such a PON is attractive because of the low installation and maintenance costs, whilst allowing the demand for new communications bandwidth services to be met. Such a PON is considered to be passive because it utilises unpowered optical splitters which enable a single optical fibre to send data to multiple users in a downstream direction. Accordingly no active elements are required between a Central Office (CO) node and the customer premises to transmit data in the downstream direction. For these reasons PONs and 10G PONs have experienced significant growth in recent years, and the trend for broadband access networks based on optic fibre is set to continue.
It is also known to provide a Wavelength Division Multiplexing PON (WDM-PON) which uses multiple optical wavelengths to increase the upstream and/or downstream bandwidth. The multiple wavelengths of the WDM-PON can be used by separate Optical Network Units (ONUs) to create several virtual PONs which co-exist on the same physical infrastructure. A problem associated with existing PONs and WDM-PONs is that active optical components, such as lasers, are often required at the user location so that user data can be transmitted in an upstream direction.
It is known from “A Novel Hybrid WDM/SCM-PON Sharing Wavelength for Up- and Down-Link Using Reflective Semiconductor Optical Amplifier”, IEEE Photonics Technology Letter, Vol 18, No 3, 1 Feb. 2006, to remodulate a downstream base-band signal as a Sub-Carrier Multiplexed (SCM) optical signal for upstream transmission. This allows the downstream and upstream signals to occupy different regions of the available spectrum. The WDM/SCM-PON achieves this by splitting the downstream base-band signal into two portions at an ONU. One of the portions is passed to a photodiode for onward transmission to the user. The other portion is passed to a Reflective Semiconductor Optical Amplifier (RSOA) where it is remodulated as a SCM signal with data from the users by using a local oscillator and operating the RSOA in a gain saturation region.
A problem associated with the WDM/SCM-PON arrangement is that a complex assortment of optical and electronic components are required at the ONU to allow the RSOA to sufficiently erase the data from the downstream base-band optical signal, to transform the upstream signal into a different region of the spectrum, and to pass the downstream data to the users. Typically the known WDM/SCM-PON requires a local oscillator at the ONU for the upstream signal, and an electrical demodulator at the ONU for the downstream signal. Overall the WDM/SCN-PON is complex and requires additional components which increases costs.
When SCM is used in the upstream direction the Radio Frequency (RF) subcarriers need to be optically modulated at the ONU using an additional Continuous Wave (CW) light-source such as a laser. Accordingly such SCM arrangements are less desirable for transmission of data in the upstream direction due to the additional stage of RF-to-optical conversion at the ONU.